1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a hard disk drive having an actuator controller that accounts for the bias torque on the actuator.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost. However, because the computer industry continually strives to provide computer systems with increased performance, there exists a need for improved disk drives having increased data access speeds.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk and data is written to or read from each track by positioning the transducer over the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical hard disk drive further comprises a head stack assembly (HSA) that includes a transducer, a pivotally mounted actuator arm for supporting the transducer, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil which results in the permanent magnet applying a force onto the coil which is then transferred to the actuator arm in the form of a torque. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to reposition the transducer by first directing the control current through the coil so as to angularly accelerate the actuator arm in a first direction and then reversing the control current so as to angularly decelerate the actuator arm.
The HSA further comprises a pivot bearing and a thin flexible cable know as xe2x80x9cflex cablexe2x80x9d. The pivot bearing provides the pivotal support of the actuator arm while allowing precise rotational motion. The flex cable provides an interconnect between the transducer and the disk control circuitry. The flex cable is attached to the actuator arm, and is allowed to flex and unflex as the actuator arm moves back and forth.
The time required to reposition the transducer in the foregoing manner is known as the xe2x80x9cseek timexe2x80x9d of the drive and is an important performance factor that affects the throughput of the drive. For example, a drive having a short seek time will be able to access a requested track of data more quickly than a drive having a longer seek time. According the state of the art, the seek time required to reposition the transducer across a distance of 0.8-0.85 cm is typically in the range of 5-10 ms.
In a typical seek operation, the transducer accelerates, coasts, and decelerates according to the predetermined control of the current applied to the VCM. The transducer, through a feedback control, typically requires some settling time to settle on the proper target track. Once the transducer is on the proper track, a track following bias current is provided to the VCM in order to maintain tracking. The bias current is necessary to counteract a bias torque that is continuously exerted on the actuator arm.
The bias torque comprises any torque acting on the actuator arm, other than the torque due to the control current in the VCM (control torque). A torque on the actuator arm from the spring property of the flex cable is one potential source of such bias torque. For example, the flex cable may be stretched when the actuator arm moves in one direction, and squeezed when the actuator arm moves in the other direction thereby affecting the control torque on the actuator arm. Friction is another factor that contributes to the bias torque on the actuator arm.
Typically, the magnitude of the bias torque is much smaller than that of the control torque. As such, the bias torque is not significant when the control torque is being applied to the actuator arm for gross movements. During the settling phase and track following, however, small amounts of currents are applied by the VCM in a prescribed manner to make the transducer settle on the target track and maintain the track following thereafter. In these realms of operation, the effects of bias torque can become significant, especially in higher density drives.
In practice the bias torque is usually not characterized analytically, but measured. The feedback control system measures the current required by the VCM to counter the bias torque on the actuator arm so as to maintain the transducer at a given track. A plurality of such measurements are made at various locations on the disk, and the resulting curve yields the bias torque as a function of track location.
The bias torque curve as a function of track location is not unique. That is, bias torque measured when the transducer is moving from inner diameter side to the outer diameter side yields a first curve. Bias torque measured when the motion is in the opposite direction yields a second curve that typically does not retrace the first curve. When a full range of motion is made from the inner most track to the outer most track, and then back to the inner most track, the resulting bias torque measurements yield two distinct sets of curves, with the two curves joined at the inner most and the outer most points. This behavior resulting in the loop shape is known as hysteresis. Hysteresis is a phenomenon where a measured quantity depends on the direction of a process.
Hysteresis in general is caused by a xe2x80x9cmemoryxe2x80x9d of a system that makes certain aspects of the system to lag behind. In the case of the actuator arm, the memory comes from the mechanical memories of either the flex cable or the pivot bearing or both. That is, a bias torque depends on the recent history of the flex cable and/or the pivot bearing. If the actuator arm comes to a stop at a given track, it will feel a certain bias torque based on the memory of the previous movements, the memory including the direction of travel.
Hysteresis in bias torque is further complicated by its dependence on the size of a seek loop. A full seek loop is when the transducer goes from the inner most track to the outer most track, and returns to the inner most track. A smaller seek loop comprising a small length seek and its reverse, yields a smaller hysteresis loop that is nested within the larger full seek loop. Progressively smaller seek loops yield smaller hysteresis loops that are nested within the larger loops.
To achieve optimal seek performance, the controller must account for the bias torque and adjust the control current to the VCM. If the proper adjustment is not made, the transducer can overshoot or undershoot the target track sufficiently to cause a delay in settling, thus increasing the seek time. Thus, there is a need to adjust the control current to the VCM according to the bias torque acting on the actuator arm.
One possible solution is to consider only the average bias torque. Averaging of the bias torque hysteresis loop will yield a single bias torque curve as a function of track location (nominal bias curve). An X-Y lookup table can be used to correlate bias torque (Y) to track number (X), and the result from this single lookup table can be used by the servo controller to adjust the control current. This method is sufficient if the hysteresis effect is small, or if the delay due to the hysteresis effect during the settling phase is within an acceptable limit.
Another solution is proposed in U.S. Pat. No. 5,872,674 to Eddy and assigned to Seagate Technology, Inc., USA. In this patent, a control system takes into account not only the track location, but also seek direction, previous seek direction, and current seek length to determine the adjustment value needed to offset the bias torque. In this method, the seek direction parameter determines whether the bias torque to be encountered is on the top or bottom of the hysteresis loop. The previous seek direction parameter determines whether the last seek operation used the bias torque offset value from the top or bottom of the hysteresis loop. The current seek length determines the magnitude of the hysteresis loop to be encountered. The location parameter determines the nominal bias torque to be encountered at the target track location.
The solution proposed by Eddy has a drawback of relying on parameters associated only with the last operation and the operation to be done. By definition, hysteresis is caused by the memory of the system, and that memory may last longer than the time elapsed between the last operation and the current operation. As an example, suppose that the actuator arm has performed a series of seek operations, all of them involving short seek lengths. It then does a long seek length operation as the most recent operation. The current seek operation to be done involves a short seek length. The memory of the Eddy system will most likely be in a state such that the system is used to doing short length seeks, and the single long seek operation will most likely not erase that memory. The control logic cannot distinguish this history from that involving a series of long length seek operations, including the most recent one. As such, the logic""s output based only on the most recent and the current operations may not be indicative of the actual bias torque. In particular, relying only on the current seek length and the most recent seek direction may not accurately represent many of the possible bias torque that the actuator arm experiences.
It is desirable to be able to determine the bias torque offset value accurately. In some disk drives high accuracy is not required in determining the bias torque offset, due to the actuator arm being sized such that the hysteresis effect is essentially negligible when compared to the moment of inertia of the actuator arm. The current trend in the disk drive market, however, is toward smaller drives. A physically smaller disk drive will have a smaller actuator arm and also a smaller VCM, such that the bias torque will become more significant when compared to the actuator arm""s moment of inertia and the control torque. Thus, an accurate system of determining the bias torque offset becomes increasingly important.
From the foregoing, it will be appreciated that there is a need for improved system for determining the bias torque offset value to improve the seek performance of the disk drive. To this end, there is a need for a system that accurately determines the bias torque offset based on the parameters of the operation to be performed, as well as the parameters from the history of the previous operations.
In one aspect of the invention, a hard disk device comprises a rotatable disk having a magnetic recording media wherein the rotatable disk defines a plurality of concentric servo tracks. The hard disk further comprises a pivotable actuator that is movable with respect to the rotatable disk. The pivotable actuator is subject to a first bias force that has a history component which varies at least in part, based upon the history of previous movements of the pivotable actuator. The hard disk further comprises a transducer positioned on the actuator so as to be movable with respect to the disk so as to be positionable on a selected servo track of the plurality of concentric servo tracks. The hard disk further comprises a controller that induces bias outputs to be applied to the pivotable actuator during positioning of the actuator. The controller induces a first bias output to be applied to the actuator to position the actuator with respect to the rotatable disk so as to position the transducer on a selected servo track of the plurality of servo tracks. The controller monitors at least one inflection length that is defined as the total number of tracks traversed during a plurality of seek operations between changes in direction of the pivotable actuator and determines the magnitude of the first bias output by determining a first magnitude value indicative of the approximate bias needed to position the actuator so as to position the transducer on the selected servo track, then offsets the first magnitude value by a history offset value that corresponds to the history component of the first bias force exerted on the actuator. The history offset is determined based upon the monitored at least one inflection length.
In one embodiment of the hard disk device, the controller induces the application of the first bias output during a seek operation that moves the transducer from a first position over a first servo track to a second position over a second servo track. The first bias output settles the actuator in the second position such that the transducer is positioned over the second servo track.
The hard disk device further comprises a memory having a plurality of location bias values stored therein. The memory is used by the controller to determine the first magnitude value of approximate bias needed to position the actuator so as to move the transducer from the first position over a first servo track to the second position over the second servo track. The memory includes a first X-Y look up table wherein the location bias values are plotted versus location.
In one embodiment of the hard disk device, the location bias value depends on the operating temperature of the hard disk device. Parameters that define temperature dependence of the location bias value are stored in the memory as a look up table.
The memory also includes a direction offset value stored therein that the controller uses to adjust the location bias value in obtaining the magnitude of the first bias output. The direction offset value is representative of the direction of movement of the actuator during the selected seek operation.
The memory further includes a plurality of history offset values stored therein that are indicative of the offset values used to offset the first magnitude value so as to at least partially offset the history component of the first bias force exerted on the pivotable actuator. The controller uses the location bias value, the direction offset value and the history offset value to obtain the magnitude of the first bias output needed for a selected seek operation.
The history offset value reduces the magnitude of the adjustment of the direction offset value to the location bias value for the selected seek operation. The memory further comprises a second X-Y look up table that correlates the history offset values versus a weighted average of the inflection lengths of the plurality of previous seek operations. The history offset value plotted versus the weighted average of the inflection lengths has a negative value for at least a portion of the second X-Y look up table so as to diminish the effect of the adjustment of the direction offset value to the location bias value. The history offset value has a zero magnitude for seek lengths greater than a pre-selected length.
In one embodiment of the hard disk device, the controller induces the application of the first bias output during track following immediately subsequent to the seek operation. The first bias output counteracts the first bias force on the pivotable actuator and maintain the transducer in a position over the servo track.
To apply the first bias output during track following, the hard disk device further comprises a memory having a plurality of location bias values stored therein that is used by the controller to determine the first magnitude value of approximate bias needed to maintain the actuator over the servo track. The memory includes the first X-Y look up table wherein the location bias values are plotted versus location.
In one embodiment of the hard disk device, the location bias value used during track following depends on the operating temperature of the hard disk device. Parameters that define temperature dependence of the location bias value are stored in the memory as a look up table.
The memory also includes a direction offset value stored therein that the controller uses to adjust the location bias value in obtaining the magnitude of the first bias output. The direction offset value is representative of the direction of movement of the actuator during the selected previous seek operation.
The memory further includes a plurality of history offset values stored therein that are indicative of the offset values used to offset the first magnitude value so as to at least partially offset the history component of the first bias force exerted on the pivotable actuator. The controller uses the location bias value, the direction offset value and the history offset value to obtain the magnitude of the first bias output needed for the selected track following operation.
The history offset value reduces the magnitude of the adjustment of the direction offset value to the location bias value for the selected track following operation. The memory further comprises the second X-Y look up table that correlates the history offset values versus a weighted average of the inflection lengths of the plurality of previous seek operations. The history offset value plotted versus the weighted average of the inflection lengths has a negative value for at least a portion of the second X-Y look up table so as to diminish the effect of the adjustment of the direction offset value to the location bias value. The history offset value has a zero magnitude for previous seek lengths greater than a pre-selected length.
Another aspect of the invention relates to a method of adjusting the position of an actuator having a transducer positioned thereon with respect to a selected servo track of a rotating hard disk. The method comprises assessing a plurality of previous seek operations wherein the actuator has moved the transducer from one servo track to another to determine a history offset value corresponding to a history component of a first bias force exerted on the actuator. The history component varies, at least in part, based on the history of previous movements of the pivotable actuator.
The method further comprises determining an initial magnitude value of a bias output needed to be applied to the actuator to position the transducer over the selected servo track. The method further comprises adjusting the initial magnitude value of the bias output by the history offset value to obtain a first bias output, and applying the first bias output to the actuator so as to position the transducer over the selected servo track.
Applying the first bias output to the actuator counteracts the first bias force acting on the actuator during settling of the transducer over the selected servo track during a seek operation. The first bias output also counteracts the first bias force acting on the actuator during track following immediately subsequent to the seek operation.
Determining the initial magnitude value of the bias output comprises determining the initial magnitude value""s dependence on the location of the selected track, and the direction of the seek operation. The initial magnitude of the bias output further depends on the operating temperature.
Assessing the plurality of previous seek operations comprises monitoring and weighted averaging at least one inflection length that is defined as total number of tracks traversed during a plurality of seek operations between changes in direction of the actuator. The weighted average of the inflection lengths is indicative of the history offset value.
From the foregoing, it should be apparent that the use of history offset value improves the performance of the hard disk during seek and track following operations. These and other objects and advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawing.